We report on the wafer scale fabrication of single-mode low-loss lithium niobate on insulator waveguides utilizing a chemically amplified resist and an optimized dry etching method. The fabricated single-mode waveguides are free of residuals and re-deposition, with measured losses for straight waveguides around 2 dB/m (0.02 dB/cm). We present a method offering advantages for large-scale production mainly due to its cost-effectiveness and faster writing time. This work holds promise for advancing integrated photonics and optical communication technologies. (c) 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

Quantum nonlinear interferometers play an important role in quantum imaging and sensing applications. Here, we demonstrate the tailoring of the frequency-angular spectrum of such a device, where we use a linear dispersive medium as a phase corrector. By changing the thickness of the dispersive medium, we tune the phase of the interferometer, enabling control over the frequency-angular spectrum and visibility of the interference fringes. Utilizing the enhanced visibility achieved through our proposed technique, we show the application of a simple interferometer in spectroscopy with an undetected photon scheme. We distinguish different polymers by measuring their characteristic absorption features in the mid-infrared spectral range above 6 μm while detecting only photons in the visible range.

Polarization-entangled photon-pair states are useful for quantum applications. Here, we propose generation of these states with controlled spectral properties using doubly periodically poled and dispersion-engineered thin-film lithium niobate (TFLN) ridge waveguides.

Sources of spectrally engineered photonic states are a key resource in several quantum technologies. Of particular importance are the so-called factorizable biphoton states, which possess no spectral entanglement and hence, are ideal for heralded generation of high-purity single photons. An essential prerequisite for generating these states through nonlinear frequency conversion is the control over the group indices of the photonic modes of the source. Here, we show that thin-film lithium niobate on insulator (LNOI) is an excellent platform for this purpose. We design and fabricate periodically poled ridge waveguides in LNOI to demonstrate group index engineering of its guided photonic modes and harness this control to experimentally realize on-chip group index matched type-II sum-frequency generation (SFG). Also, we numerically study the role of the top cladding layer in tuning the dispersion properties of the ridge waveguide structures and reveal a distinctive difference between the air and silica-clad designs which are currently among the two most common device cladding configurations in LNOI. We expect that these results will be relevant for various classical and quantum applications where dispersion control is crucial in tailoring the nonlinear response of the LNOI-based devices.

Quantum imaging is an ever expanding research field in which the aim is to exploit the quantum nature of light to enhance image reconstruction capabilities. Despite a number of successful demonstrations for quantum imaging, quantum microscopy still seems out of the range for practical applications due to different physical and technical reasons. Here, we propose an imaging method exploiting the quantum correlations of photon pairs and a scanning microscope to achieve fast, single mode quantum imaging. We first test our technique on a metal grating to estimate the resolution capabilities of our system. Moreover, we assess its potential in terms of the number of available independent pixels at full resolution compared to different quantum imaging approaches. Finally, we demonstrate scanning quantum microscopy of onion epithelial cells, paving the way toward scalable quantum microscopy for bio-physical applications. Our results, combined with the rapidly evolving photon-pair generation and detection technology toward the mid-infrared, could lead to an extension of quantum microscopy applications toward the mid-infrared to access the molecular fingerprint region.

We propose a novel quantum nonlinear interferometer design that incorporates a passive parity – time (PT)-symmetric coupler sandwiched between two nonlinear sections where signal – idler photon pairs are generated. The PT symmetry enables efficient coupling of the longer-wavelength idler photons and facilitates the sensing of losses in the second waveguide exposed to analyte under investigation, whose absorption can be inferred by meas-uring only the signal intensity at a shorter wavelength where efficient detectors are readily available. Remarkably, we identify a new phenomenon of sharp signal intensity fringe shift at critical idler loss values, which is distinct from the previously studied PT symmetry breaking. We discuss how such unconventional properties arising from quantum interference can provide a route to enhancing the sensing of analytes and facilitate broadband spectroscopy applications in integrated photonic platforms.

Lithium niobate (LN) is a promising and versatile material platform for implementing various elements essential for realizing integrated photonic technology. We report on integrated entangled photon-pair sources and adiabatic coupler circuits built in bulk/thin-film LN. © 2021 The Author(s)

Nanoscale periodically poled lithium niobate (LiNbO 3 ) waveguides with cross-sectional area below 1 μm 2 have recently enabled nonlinear frequency up-conversion of light with ultrahigh efficiencies [1] . Such ultra-compact nanophotonic waveguides also allow extensive control over the dispersion properties of their guided modes which has been exploited to realize broadband phase-matched nonlinear optics and supercontinuum generation [2] . In this work we experimentally demonstrate type-II quasi phase-matched sum frequency generation (SFG) in a dispersion engineered nanowaveguide. We explicitly show that the phase matching condition for SFG can be tailored by controlling the group indices of the guided modes through optimal design of the waveguide. Also, we contrast the phase matching condition for this type-II process with that of a type-0 mode-matched SFG process occurring in the same waveguide to clearly distinguish the role of dispersion engineering in nonlinear wavelength conversion processes in nanowaveguides.

X iv :2 11 0. 01 51 1v 1 [ ph ys ic s. op tic s] 4 O ct 2 02 1 Mid-Infrared Photon-Pair Generation in AgGaS2 Mohit Kumar, a) Pawan Kumar, Andres Vega, Maximilian A. Weissflog, 2 Thomas Pertsch, 2, 3 and Frank Setzpfandt Institute of Applied Physics, Abbe Center of Photonics, Friedrich Schiller University Jena, Albert-Einstein-Str. 15, 07745 Jena, Germany Max Planck School of Photonics, Albert-Einstein-Str. 6, 07745 Jena, Germany Fraunhofer Institute for Applied Optics and Precision Engineering, Albert-Einstein-Str. 7, 07745 Jena, Germany

We implement a nondegenerate polarization-correlated photon-pair source on titanium-diffused nonperiodically-poled lithium niobate waveguides. The nonperiodic domains are optimized using genetic algorithm to maximize and equalize efficiencies of the spontaneous parametric down-conversion processes.

We present a generalized understanding of the induced-coherence (IC) effect, aiming to find new strategies for engineering and optimizing the IC response of nonlinear systems. We establish that sensing the cross density of states (CDOS) of the field lies at the core of IC and that it is the spatial profile of the nonlinearity that determines how this CDOS information is sampled. Based on our findings, we identify integrated nonlinear waveguides as a versatile and suitable platform for spectroscopy based on IC and show that our generalized treatment allows us to optimize the sensing performance. Our results open the way for the design of compact IC-based spectroscopic devices with customized responses.

Direct spectral characterization of a quantum photon-pair source usually involves cumbersome, costly, and time-consuming detection issues. In this study, we experimentally characterize the spectral properties of a type-II phase-matched spontaneous parametric down-conversion (SPDC) source based on a titanium-diffused periodically poled lithium niobate (Ti:PPLN) waveguide. The characterization of the spectral information of the generated cross-polarized photon pairs is of importance for the use of such sources in applications including quantum information and communication. We demonstrate that the joint spectral intensity of the cross-polarized photon-pair source can be fully reconstructed using the quantum-classical correspondence through classical sum-frequency generation (SFG) measurements. This technique, which uses a much less complex detection system for visible light, opens the possibility of fast monitoring and control of the quantum state of (polarization-correlated) photon-pair sources to facilitate the realization of a stable and high-usability quantum source.

Summary form only given. Induced coherence (IC) between two spatially separated but indistinguishable spontaneous parametric down conversion (SPDC) processes underpins the recent revival of nonlinear interferometry and has led to new applications in quantum imaging and mid-infrared (MIR) spectroscopy [1,2]. These applications exploit induced coherence and the spectral or spatial correlations between signal and idler photons generated through SPDC. Here, we theoretically describe the phenomenon of IC in a general way by making use of a quantum mechanical approach based on the classical Green's function (GF) of the system [3,4]. Our formalism is well suited for treating generic linear optical properties of a system, including loss, and studying their influence on the IC phenomena. Moreover, our approach can account for arbitrary spatial profiles of the nonlinearity and hence can be used to describe generalized IC experiments with different number of photon-pair sources. Here we propose a sensing experiment using a single nonlinear waveguide [5] and investigate IC using our formalism. We find that in this case a substance under test inside or surrounding the waveguide can affect the IC during the generation stage along the nonlinear waveguide, making this a compact scheme for MIR spectroscopy without the need for MIR detectors.

Quantum spectroscopy based on nonlinear interferometers attracts increasing attention due to its unique practical advantages [1]. By tailoring spontaneous parametric down conversion (SPDC) for generation of frequency-correlated photons and their nonlocal quantum interference, it becomes possible to perform sensing and imaging across broad spectral ranges through simple detection of paired photons in the visible [2,3]. It was recently demonstrated that nonlinear integrated waveguides can improve the performance of quantum spectroscopy [4]. However, while fabrication of integrated circuits with multiple waveguides is well established, their applications for quantum spectroscopy remain largely undeveloped.

We experimentally characterize the spectral properties of photon pairs generated by type-II spontaneous parametric down-conversion waveguide. The spectral properties of cross-polarized photon-pair are determined by quantum-classical correspondence.

Lithium niobate (LN) is a favourable material for many applications, especially in integrated optics, due to its excellent electro-optic, acousto-optic, and nonlinear optic properties. One promising application of LN is second-harmonic generation (SHG). For enhancing the efficiency of SHG, phase matching between the interacting waves is needed, which is often realized by employing quasi-phase matching (QPM) [1]. QPM is typically realized by periodic poling of LN [2,3], i.e. the periodic flipping of the crystal orientation. The needed period is determined by the wavelengths as well as by the wave vectors of the fundamental- and the second-harmonic waves (FW and SH, respectively). For the case of backward SHG with a fundamental wavelength of 1.55 μm, where the SH wave travels in the opposite direction to the FW, typically sub-micrometer periods of the poled LN are required [4].

Spontaneous parametric down-conversion (SPDC) spectroscopy using photon pairs is a promising avenue towards affordable mid-infrared (MIR) spectroscopy. Here, we experimentally investigate the feasibility of using periodically poled waveguides in lithium niobate for SPDC spectroscopy applications. We find the waveguides suitable to generate wavelength non-degenerate photon pairs with one photon in the MIR spectral range with high fluence. We use this to determine the cutoff wavelengths of the waveguide mode in the MIR by performing only measurements in the near-infrared spectral range.

We demonstrate experimentally on-chip-integrated spontaneous parametric down-conversion spectroscopy by generating biphotons in a LiNbO3 waveguide and using signal photon detection in the NIR to study the dynamics of idler photons in the MIR.

We demonstrate experimentally on-chip-integrated quantum spectroscopy by generating biphotons in a LiNbO3 waveguide through spontaneous parametric down-conversion, and using signal photon detection in the NIR to study the dynamics of idler photons in the MIR.